As integrated circuits become denser by making the circuit components smaller and packing them closer, it becomes increasingly important to interconnect the circuit components into an integrated circuit by conductors of as low a resistance as practical. To this end, there is increasing interest in the use of copper as the metal of choice for providing such interconnections because of both favorable electrical properties and relatively low cost. Copper has proven of particular interest when used as the back-end-of-line metal to provide the final metal interconnection, particularly as part of a single or dual Damascene overlying the process. In this process a pattern of trenches is formed in an insulating layer overlying the chip in which were formed the circuit componenets to be interconnected. The pattern of trenches is then overfilled by depositing a copper layer over the surface of the insulating layer and the copper is then removed from the layer except where it has filled the pattern. The removal is typically done by chemical mechanical polishing (CMP). Generally it is necessary to work with trench patterns that comprise relatively narrow lines because of space considerations. To achieve high conductance (low resistance) in view of the use of narrow width trenches, it is desirable to make the trenches relatively deep.
Presently preferred technology for filling with copper trenches of a width of between 0.15 and 0.25 microns and a depth of about 0.3 to 0.4 microns involves a sequence of steps. These include first sputtering (physical vapor deposition) a barrier-glue liner layer over the walls of the trench to be filled, then sputtering thereover a seed layer of copper, and finally completing the filling by electroplating copper. However, technology of this kind has proven to be relatively ineffective for essentially complete copper fill of trenches of aspect ratios much higher than five or six.
The term "trench" is used herein to describe both an extended groove and a localized via a region that extends from the bottom of the groove to a localized connection to an underling conductive region, such as a region of the semiconductor chip or another conductive line.
The present invention seeks to provide technology that can provide a satisfactory copper fill of trenches with aspect ratios significantly higher than 5 or 6, such as 10 or 12.
Another problem with copper is the issue of reliability. In particular, it is important that the conductivity of the copper fill remain relatively constant over long periods of use and not suffer from an ill described as "stress migration", which results in a change, usually an increase, in the resistance of the fill. Again present technology still has room for improvement in this respect. The present invention as another feature improves reliability especially with reducing stress migration.